Area Forecast Discussion
Issued by NWS Chicago, IL

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000
FXUS63 KLOT 220312
AFDLOT

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL
1012 PM CDT THU MAY 21 2015

.UPDATE...
759 PM CDT

ONLY MINOR ADJUSTMENTS HAVE BEEN MADE TO THE GOING FORECAST. A
SOMEWHAT WEAK UPPER WAVE...BUT AT EDGE OF A STRONG POLAR LOBE
WITH -37C OBSERVED AT 500MB IN ONTARIO...IS MOVING SOUTHEAST
ACROSS CENTRAL LAKE MICHIGAN. AN ASSOCIATED SURFACE COLD FRONT IS
DRAPED WEST-TO-EAST ACROSS CENTRAL WI AND ACCELERATING DOWN THE
WESTERN SHORE OF LAKE. CONVERGENCE WITH THIS FRONT AND THE STEEP
LAPSE RATES ALOFT AS SAMPLED BY THE 00Z GRB SOUNDING HAVE AIDED IN
SCATTERED SHOWERS ACROSS EASTERN WI AND CENTRAL LAKE MI EARLY
THIS EVE. THESE ARE STRUGGLING WITH VERY DRY AIR EVIDENCED BY THE
GRB SOUNDING AND JUST THE NATURE ON SATELLITE AND RADAR MOSAIC
LOOPS. IT IS POSSIBLE FAR NORTHEAST ILLINOIS AND NORTHWEST INDIANA
GET GRAZED WITH SOME SPRINKLES BETWEEN 10 PM AND 2 AM. A MORE
NOTEWORTHY FEATURE WILL BE THE TEMPERATURE DROP BEHIND THE
FRONT...SINCE READINGS ARE LIKELY TO ONLY SLOWLY FALL UNTIL FROPA
AND THEN SHARPLY DROP...ESPECIALLY NEAR THE LAKE FRONT. READINGS
SHOULD BE IN THE 40S NEAR THE LAKE FRONT WITHIN 1-2 HOURS AFTER
FROPA...AND 40S ARE FORECAST AREAWIDE FOR DAYBREAK ON FRIDAY.

MTF

&&

.SHORT TERM...
247 PM CDT

THROUGH SATURDAY...

HIGH PRESSURE IS OVER THE PLAINS AND SPREADING ACROSS CENTRAL AND
SOUTHERN IL.  EARLY MORNING CLOUDS HAVE CLEARED NICELY AND
TEMPERATURES ROSE QUICKLY IN RESPONSE TO THE AMPLE SUNSHINE.  A WEAK
LOW PRESSURE TROUGH AND COLD FRONT ARE OVER THE NORTHERN END OF LAKE
MICHIGAN AND THE FRONT WILL SLIDE DOWN THE LAKE OVERNIGHT. WHILE A
FEW LIGHT RAIN SHOWERS ARE EXPECTED WITH FRONT OVER THE
LAKE...THINKING IT WILL BE TOO DRY FOR PRECIP TO FALL OVER THE LAND
BASED CWA.  WINDS BECOME NORTH BEHIND THE FRONT LATE TONIGHT INTO
FRIDAY MORNING.

WINDS DROP OFF QUICKLY AS HIGH PRESSURE BUILDS OVERHEAD. FLOW WILL
BE OFF THE LAKE AND EXPECTING WINDS AROUND 10 MPH ALONG THE LAKE
FRIDAY.  AREAS WELL AWAY FROM THE LAKE WILL GET INTO THE MID 60S TO
AROUND 70...WHILE AREAS ALONG THE LAKE WILL ONLY BE IN THE 50S TO
LOW 60S.  OUTSIDE OF CHILLIER TEMPS ALONG THE LAKE...EXPECTING A
BEAUTIFULLY SUNNY DAY. OVERNIGHT LOWS WILL VARY FROM THE LOW 40S TO
AROUND 50.

THE HIGH SHIFTS EAST SATURDAY BUT EXPECTING ANOTHER DRY DAY WITH
INCREASING CLOUD COVER.  THE NEXT LOW FORMS OVER THE CENTRAL PLAINS
TOMORROW BUT THE UPPER LEVEL RIDGE REMAINS OVER THE MIDWEST.
THINKING THE DRY AIR WILL KEEP THE REGION PRECIP FREE THROUGH
SATURDAY AFTERNOON. SOUTH WINDS ARE EXPECTED AND A WEAK LAKE BREEZE
IS POSSIBLE. THINKING THE LAKE BREEZE WILL HUG THE SHORE AND
PROBABLY ONLY PUSH OVER LAND OVER LAKE COUNTY IL.  AS SUCH KEPT
DAYTIME HIGH TEMPS IN THE 60S ALONG THE LAKE BUT EXPECTING ALL OTHER
REGIONS TO CLIMB INTO THE MID 70S. IF CLOUD COVER IS SLOWER THAN
FORECAST...COULD SEE A FEW SITES REACH 80...BUT DECIDED TO GO THE
SAFER ROUTE AND BET ON INCREASING CLOUD COVER. HAVE MEDIUM-HIGH
CONFIDENCE IN HIGH TEMPS FOR SATURDAY.

JEE

&&

.LONG TERM...
307 PM CDT

SATURDAY NIGHT THROUGH THURSDAY...

PRETTY BIG MODEL DISCREPANCIES CREEPING IN WITH RESPECT TO THE
PATTERN EVOLUTION ON SATURDAY NIGHT AND ESPECIALLY SUNDAY AS
WESTERN TROUGH OPENS UP AND OUR AREA ON WESTERN PERIPHERY OF
EASTERN RIDGE. MUCH OF SATURDAY NIGHT NOW APPEARING IT COULD BE
DRY...ESPECIALLY PER GFS/GEFS/GEM/NAM BUT DID KEEP SLIGHT POPS IN
FAR WEST AFTER MIDNIGHT. ON SUNDAY...MODELS SHOW BEST FORCING FROM
AN UPPER WAVE FOCUSING INTO NORTHWEST IL AND SOUTHERN
WISCONSIN...BUT THE SIGNIFICANT DIFFERENCES BETWEEN GFS AND ECMWF
ARISE BECAUSE GFS FIRMLY ENTRENCHES US IN THE WARM SECTOR WHILE
ECMWF FOCUSES WIDESPREAD SHOWERS ON CONVERGENCE NOSE OF 850 MB
JET. FORECAST IS A BIT OF A MIDDLE GROUND...WITH LIKELY POPS IN
WESTERN HALF OF CWA CLOSER TO UPPER WAVE FORCING AND CHANCE POPS
TO THE EAST. DUE TO POOR MIDLEVEL LAPSE RATES ON ALL THE GUIDANCE
HAVE ONLY INCLUDED A SLIGHT CHANCE FOR TSRA. IF LESS CLOUDS/PRECIP
OCCUR...THEN LOWER-MID 70S TEMPS FORECAST FROM IL SHORE WOULD
LIKELY BE TOO COOL...WHILE ON OTHER HAND...MUCH COOLER TEMPS WOULD
RESULT FROM A EC LIKE EVOLUTION. CONFIDENCE IS LOW IN THE SUNDAY
FORECAST.

FOR MEMORIAL DAY THROUGH WEDNESDAY...MULTIPLE DISTURBANCES RIDE
INTO AREA IN WEST-SOUTHWEST FLOW ALOFT...SO HAVE POPS THE ENTIRE
PERIOD. HOWEVER...IT DOES NOT APPEAR ANY DAY WOULD BE A WASHOUT.
WE DO GET INTO BETTER INSTABILITY DUE TO MIDLEVEL LAPSE RATES AND
60S DEWPOINTS ALONG WITH MODERATE MIDLEVEL FLOW. A SURFACE LOW
LOOKS TO LIFT NORTHEAST ON MONDAY...BUT BIG VARIANCE IN HOW CLOSE
TO THE AREA IT WILL GET. ANOTHER DISTURBANCE COULD THEN LIFT
TOWARD THE REGION TUESDAY INTO WEDNESDAY. CONFIDENCE IS AGAIN LOW
GIVEN HOW DIFFICULT IT IS FOR GUIDANCE TO HANDLE TIMING/ETC OF
DISTURBANCES IN ACTIVE WSW FLOW PATTERN. THERMAL PROGS ARE QUITE
WARM WITH 850 MB TEMPS IN MID TEENS CELSIUS AND 925 MB TEMPS IN
HIGH TEENS CELSIUS...SO AGAIN TEMPERATURES COULD END UP WARMER
THAN CURRENTLY FORECAST IF MORE SUN IF OBSERVED. STILL WILL LIKELY
BE A MUGGY PERIOD GIVEN HIGHER DEWPOINTS AND ABOVE SEASONAL TEMPS.
THEN BY THURSDAY...LATEST ECMWF KEEPS US IN SOUTHERLY SFC FLOW AND
SIMILAR UPPER PATTERN WHILE OPERATIONAL GFS BRINGS DOWN HIGH
PRESSURE BEHIND WEDNESDAY SYSTEM. STAYED CLOSE TO CONSENSUS BLEND
DUE TO LARGE UNCERTAINTY.

RC

&&

.AVIATION...

//ORD AND MDW CONCERNS...UPDATED 03Z...

* WIND SHIFT TO NORTHEAST AROUND 10KT AFTER MIDNIGHT.
* LAKE BREEZE EARLY FRIDAY AFTERNOON.
* SLIGHT CHANCE OF SHRA LATE THIS EVENING. CMS

//DISCUSSION...UPDATED 00Z...

A COLD FRONT ACROSS CENTRAL WI AND NORTHERN LOWER MI WILL MOVE
SOUTH THIS EVENING AND MOVE ACROSS THE TERMINALS AFTER MIDNIGHT...
LIKELY IN THE 06Z-07Z TIME FRAME. SHOULD BE ABLE TO GET BETTER
TIMING WITH THE 03Z UPDATES AS THE FRONT APPROACHES NORTHERN IL
LATER THIS EVENING. WEST/NORTHWEST WINDS WILL DIP TO
WEST/SOUTHWEST THIS EVENING...WITH SPEEDS DIMINISHING UNDER 10
KTS. WINDS WILL SHIFT NORTHEAST BEHIND THE FRONT AND LIKELY
INCREASE INTO THE 10-15KT RANGE...THEN SETTLE UNDER 10KTS
OVERNIGHT. WHILE WIND DIRECTIONS WILL LIKELY REMAIN NORTHEAST
FRIDAY MORNING...THEY SHOULD BE LIGHT AS HIGH PRESSURE SETTLES
OVER THE REGION. THIS WILL ALLOW A LAKE BREEZE TO DEVELOP AND
MOVE INLAND EARLY FRIDAY AFTERNOON. HIGH CONFIDENCE FOR A LAKE
BREEZE AND THAT IT WILL MOVE WEST OF ORD/MDW...BUT MEDIUM FOR
TIMING AND SPEEDS...WHICH WILL PROBABLY BE AROUND 10KTS FOR AN
HOUR OR TWO THEN SETTLE BACK UNDER 10KTS BY LATE AFTERNOON.

SCATTERED SHOWERS ACROSS EAST CENTRAL WI WILL CONTINUE TO MOVE
SOUTHEAST THIS EVENING AND SHORT TERM GUIDANCE IS FAIRLY
CONSISTENT SHOWING THIS ACTIVITY GETTING CLOSE TO NORTHEAST IL
IN THE 05Z-07Z TIME PERIOD...THOUGH REMAINING MAINLY EAST OF THE
TERMINALS AND HAVE CONTINUED WITH A DRY FORECAST FOR NOW. CMS

//ORD AND MDW CONFIDENCE...UPDATED 03Z...

* HIGH FOR WIND SHIFT AFTER MIDNIGHT...MEDIUM FOR TIMING.
* HIGH FOR LAKE BREEZE FRIDAY AFTERNOON...MEDIUM FOR TIMING.
* LOW FOR SHRA LATE THIS EVENING. CMS

//OUTLOOK FOR ORD/MDW FOR 12Z SATURDAY-12Z THURSDAY...UPDATED 00Z...

* SATURDAY...DRY/VFR. SOUTH WINDS.
* SUNDAY...SHRA LIKELY/CHANCE OF TSRA. MVFR POSSIBLE. SOUTH WINDS.
* MONDAY AND TUESDAY...CHANCE OF SHRA/TSRA. STRONG/GUSTY
  SOUTHWEST WINDS.
* WEDNESDAY...CHANCE OF SHRA/TSRA. SOUTHWEST WINDS.

&&

.MARINE...
247 PM CDT

A COLD FRONT IS OVER THE NORTHERN END OF THE LAKE AND IT WILL SINK
SOUTH ACROSS THE LAKE. EXPECTING IT TO CLEAR THE SOUTHERN END OF THE
LAKE EARLY FRIDAY MORNING...WITH WINDS BECOMING NORTH BEHIND THE
FRONT.  COULD SEE A FEW GUSTS TO 20-25 KT IMMEDIATELY BEHIND THE
FRONT. WINDS DIMINISH TO 5 TO 10 KT FRIDAY MORNING AS A SURFACE HIGH
SPREADS OVER THE REGION.  EXPECTING ONSHORE FLOW ACROSS THE LAKE
FRIDAY AFTERNOON WITH GUSTS ARND 15 KT PSBL. SOUTH WINDS SET UP
BEHIND THE HIGH FRIDAY NIGHT AND SATURDAY. COULD SEE GUSTS TO 25 KT
OVER THE NORTHERN HALF OF THE LAKE SATURDAY. A WEAK LAKE BREEZE IS
ALSO PSBL ALONG THE IL SHORE SATURDAY AND MAYBE SUNDAY.  HAVE HIGHER
CONFIDENCE IN SATURDAY/S LAKE BREEZE THAN SUNDAY/S. SOUTH WINDS ARE
EXPECTED THROUGH AT LEAST MONDAY WITH SPEEDS UP TO 20 KT.

JEE

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...NONE.
IN...NONE.
LM...NONE.
&&

$$

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